1. Field of the Invention
The present invention relates to an ink jet printer. More particularly, the invention relates to an ink jet printer of the transfer type in which an ink image is first formed on an intermediate recording medium, and then transferred from the intermediate recording medium onto an image recording medium.
2. Prior Art Discussion
The transfer type ink jet printer is reliable in that it is free from the nozzle clogging problem arising from paper dust generated when the print heat comes in contact with a print paper. An example of this type of the printer is disclosed in Japanese Patent Laid-open Publication No. Hei. 1-226336.
In the construction of the transfer type ink jet printer, a print heat with a plural number of nozzles is disposed confronting with a tubular or drum-shaped intermediate recording medium, or a transfer drum. In the print operation of the printer, the print heat sets forth a jet of ink droplets toward the intermediate recording medium, thereby to depict an image on the intermediate recording medium. The image formed thereon is dried by a heater. An image recording medium, e.g., a print paper, is pressed against the image bearing surface of the intermediate recording medium, so that the ink image is transferred from the intermediate recording medium to the image recording medium. Accordingly, the ink image of the intermediate recording medium can be transferred without any deformation, and the transferred ink image will not blur on the printer paper. Thus,the pressure image transfer process, which thus causes no deterioration of print quality, can exactly transfer the ink image from the intermediate recording medium to the recording medium.
The study to find the causes of the nozzle clogging and improper discharge of ink from the print heat in the transfer type ink jet printer was made by the inventors of the present Patent Application. The study showed the fact that the heat left in the heater used for drying the ink image on the intermediate recording medium after the printer is continuously operated causes the nozzle clogging and instable ink discharge.
In the printer construction disclosed in the above patent publication, since the surface region of the intermediate recording medium is heated to dry the image formed thereon, heat is accumulatively stored in the heater with progress of the continuous operation of the printer. The heater, when having the accumulative storage of heat, continues the heating operation after the power supply to the heater is shut off, because the heater is conditioned for continuing the heating operation of the intermediate recording medium. Since the print heat is located in close proximity to the intermediate recording medium, it is excessively heated to cause water as main solvent of ink to evaporate through the nozzle openings. Consequently, the viscosity of the ink in the print heat increases. The sticky ink clogs the nozzles of the print head, and the discharge of ink from the nozzles is abnormal or irregular.
The conventional printer of the transfer type in which the dried ink image is transferred from the intermediate recording medium to a print paper was experimentally examined and analyzed. The results of the examination and analysis were as follows. 1) The pressure to transfer the image is excessive, so that the device size and the cost to manufacture are increased. 2) Attempt to reduce the image transfer pressure forms an insufficient solid ink image on the print head. That is, the ink image transferred onto the print paper is poor in water proof and wear proof.
In the construction of Japanese Patent Laid-open Publication No. Hei, 1-226336, the dried ink image on the intermediate recording medium becomes solid. However, a large pressure is required in order to secure a firm sticking of the ink image firmly to the print paper. Therefore, the printer must be designed so as to be able to apply a large pressure for image transfer.
To obtain an excellent transfer characteristic under a small pressure, the ink image on the transfer medium, when transferred, must be put in a semisolid state by drying the image. The ink image transferred on the print paper still contains a slight amount of water. Such an image, when touched with the finger, becomes blurred. Thus, the conventional techniques suffers from the problems of the wear and water proof. To improve the solidity of the transferred image, water must be completely removed from the transferred image. To this end, the print paper having the image printed thereon must be heated as in the conventional ink jet printer of the nontransfer type. In the printer construction having a first heating means for drying the ink image on the intermediate recording medium and a second heating means for completely removing water from the image transferred on the print paper, the heating rise time of the heating means used must be short, limiting freedom of selecting the heating means. In a case where the heating means of long heating rise time is used in this print construction and the first and second heating means are operated in a continuous manner, a situation where both the heating means are in an ON state takes place inevitably. In this situation, temperature within the printer rises excessively, and the print heat is also overheated. Water of the ink in the print heat evaporates, the nozzles of the print head are possibly clogged with dried ink, and jets of ink droplets are irregularly and abnormally discharged from the nozzles of the print head.